The key feature that must be present for a chemical equation to be classified as a combustion reaction is the presence of oxygen (O2) as a reactant.

Here's why:

* Combustion is a chemical process that involves the rapid reaction between a substance with an oxidant, usually oxygen, to produce heat and light.

* The reaction releases energy in the form of heat and light, making it an exothermic process.

* The products of combustion typically include carbon dioxide (CO2) and water (H2O), along with other possible products depending on the reactants.

Example:

The combustion of methane (CH4) can be represented by the following equation:

CH4 + 2O2 → CO2 + 2H2O

In this equation, methane reacts with oxygen to produce carbon dioxide and water. This equation clearly shows the presence of oxygen as a reactant, confirming it's a combustion reaction.